Distortion in a digital-to-analog converter (DAC) can result from parasitic effects of the components that form the DAC. Due to the parasitic effect in the DAC, harmonic signals and image signals may be generated that cause distortions in the signals transmitted downstream from the DAC. The distortion introduced by the DAC may be classified as linear or non-linear. Equalizers at the receiver may attenuate the effects of linear distortions. However, non-linear distortions such as harmonic signals and image signals are more difficult to address. Image signals may be those that are above the DAC sampling frequency, or may be signals that may result from DAC defects, for example, parasitics within the DAC, that causes spurious frequency components of the input signal that are scaled down and at arbitrary frequencies.
In a digital media cable communication implementation, interoperability standards, such as the DOCSIS/DRFI standard, exist to provide guidance as to the form of the data to be transmitted to a receiver. The DOCSIS standard is more stringent for fewer channels and, as the number of channels increases, the standard becomes less stringent. In particular, the requirement on the Spurious-Free-Dynamic-Range (SFDR) and the Adjacent-Channel-Leakage-Ratio (ACLR) becomes less stringent as the channel count goes up. Therefore, the digital signal input to the DAC might need to be further processed to substantially eliminate the harmonic and image signals that cause signal distortions.
Field programmable gate array (FPGA) devices and/or application specific integrated circuits (ASICS) are used to apply the digital processing techniques to the digital signals prior to conversion by the DAC for downstream transmission. FIG. 1 illustrates a conventional FPGA or ASIC configuration for providing digital data to a radio frequency (RF) DAC.
The digital signals are processed as complex domain signals that include components having real and imaginary representations. The complex domain data signal is combined by mixer 105 with a digital modulation signal that frequency shifts the data into data blocks of, typically, 6 or 8 MHz data. The 6 or 8 MHz data is combined into several channels by combiner 120, and remains as complex domain data. The combined data is mixed at mixer 107 with another digital modulation signal from up-conversion digital modulator 130. The digital modulation signal from up-conversion digital modulator 130 is at the carrier frequency at which the digital data will be transmitted.
The output of mixer 107 is still a complex domain signal. The complex domain signal output from the mixer 107 is transformed into a data signal including only the real signal components by real domain device 140. The real domain device 140 outputs the real domain data signal to the channel combiner 150. The channel combiner 150 combines the real signals into a block of upconverted channels that are transmitted to a DAC. The described system 100 merely provides data to a DAC and does not actively attempt to attenuate or eliminate the harmonics or image signals introduced by the DAC.
The inventors have recognized the benefits of introducing a pre-distortion into the signal transmitted over the channel to “cancel” (or attenuate) the effects of the non-linear channel distortions prior to inputting the signals to a DAC. The inventive system and method may be used in the above contexts, but as well in other applications that require non-linear harmonic distortions and/or image signal distortions in a transmission signal to be addressed. For example, the inventive system and method may be used in a wireless infrastructure (WIFR) base station where stringent emission specifications are to be met.